text: Add delete_surrounding_text to protocol

Add delete_surrounding_text event in the text_model interface and the
request in the input_method_context interface. Implement it in the
example editor client and in the example keyboard so that the backspace
key works with it.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>

protocol/input-method.xml

@@ -49,6 +49,10 @@
       <arg name="text" type="string"/>
       <arg name="index" type="uint"/>
     </request>
+    <request name="delete_surrounding_text">
+      <arg name="index" type="int"/>
+      <arg name="length" type="uint"/>
+    </request>
     <event name="surrounding_text">
       <description summary="surrounding text event">
         The plain surrounding text around the input position. Cursor is the

protocol/text.xml

@@ -84,6 +84,10 @@
       <arg name="text" type="string"/>
       <arg name="index" type="uint"/>
     </event>
+    <event name="delete_surrounding_text">
+      <arg name="index" type="int"/>
+      <arg name="length" type="uint"/>
+    </event>
     <event name="preedit_styling"/>
     <event name="key"/>
     <event name="selection_replacement"/>